In general, the flux used for soldering has positive effects for chemically removing a metal oxide that resides at a solder alloy and a metal surface of a bonding object to be soldered and for allowing metallic elements to move at a boundary therebetween. Therefore, by soldering with the flux, an intermetallic compound can be formed between the solder alloy and the metallic surface of the bonding object to accomplish a strong bonding.
In the soldering using a spherical solder alloy called a solder ball, the soldering is executed by applying the flux to an electrode on a substrate, mounting the solder ball on the electrode to which the flux is applied, and melting the solder by heating the substrate in a heating furnace called a reflow furnace.
The solder paste is a complex material obtained by mixing the solder alloy powder and the flux. In the soldering using the solder paste, the soldering is executed by applying the solder paste to a portion to be soldered such as an electrode on a substrate, mounting a part on the portion to be soldered to which the solder paste is applied, and melting the solder by heating the substrate in a reflow furnace.
For example, in a soldering process using a Sn—Ag based solder alloy, a temperature profile is set so as to increase a temperature of the reflow furnace to 220° C., which is a melting temperature of the solder alloy, or higher. In general, a peak temperature, which is the upper limit temperature in the reflow furnace in the soldering process, is set to be about 240-250° C.
The flux comprises a solid component, a solvent for dissolving the solid component and the like. However, in the flux comprising conventional components, the solvent component and the like in the flux are volatilized by the heating during the soldering.
When the solvent component and like in the flux are volatilized by the heating during the soldering, gas flux fumes occur. The flux fumes adhere to places, such as a wall surface or a cooling zone in the reflow furnace, having a temperature lower than that during the heating, and then they become liquid.
Thus, when the volatilized components of the flux adhere in the reflow furnace and they become liquid, there is a possibility such that the liquid falls in drops to a product carried in the reflow furnace. Therefore, a constant cleanup work is required.
A water-soluble adhesive has been proposed for temporally fixing a solder ball having a small diameter to an electrode (refer to, for example, Patent Document 1). However, it has not considered any volatilization volume thereof at all and its composition is assumed under a condition that the solvent is volatilized.